In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Serving Gateways, or SGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and SGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Cellular Circuit-Switched (CS) telephony was introduced in the first generation of mobile networks. Since then CS telephony has become the largest service in the world with approximately 4 billion subscriptions sold. Even today, the main part of the mobile operator's revenue comes from the CS telephony service (including Short Message Services (SMS)), and the 2G GSM networks still dominate the world in terms of subscriptions. 3G subscriptions are increasing in volume, but that increase is less in part because of users with handheld mobile terminals migrating from 2G to 3G and more as a result of mobile broadband implemented via dongles or embedded chipsets in laptops.
The long-term evolution (LTE) project within 3GPP aims to further improve the 3G standard to, among other things, provide even better mobile broadband to the end-users (higher throughput, lower round-trip-times, etc.).
A common view in the telecommunication industry is that the future networks will be all-IP networks. Based on this assumption, the CS domain was removed in the LTE work. As a result, the telephony service cannot be used by a 3GPP Release 8 compliant LTE terminal, unless one of the following four things is done:    (1) Implement circuit switched (CS) fallback (CSFB), so that an LTE terminal falls back to 2G GSM when telephony service is used.    (2) Implement 3GPP IP Multimedia Subsystem (IMS)/Multimedia Telephony (MMTel), which is a simulated CS telephony service provided over IP and IMS that inter-works with the Public Switched Telephone Network (PSTN)/Public Land Mobile Network (PLMN).    (3) Implement a tunneling solution with Unlicensed Mobile Access (UMA)/Generic Access Network (GAN) over LTE where the CS service is encapsulated into an IP tunnel.    (4) Implement a proprietary Voice over IP (VoIP) solution with PSTN/PLMN interworking.
All of these four possibilities have drawbacks. In deployed GSM networks that do not have Dual Transfer Mode (DTM) capabilities; CS and Packet Switched (PS) services cannot be used in parallel. Hence, all PS services running prior to a call to or from a terminal using Circuit Switched Fallback (CSFB) are put on hold or are terminated. If the GSM network has DTM, the PS performance will be greatly reduced (from 10's of Mbps to 10's to 100's of kbps). One drawback with the CS fallback approach is that when calling or being called and the terminal is falling back to GSM and the CS service from LTE. Circuit Switched Fallback (CSFB) also prolongs call set-up time.
The IMS/MMTel approach uses a completely new core/service layer that is IMS based. This provides new possibilities to enhance the service but also comes with the drawback of a financial hurdle for the operator to overcome. A new core network drives capital expenditures (CAPEX), and integration of that core network drives an initial operating expenditures (OPEX) increase. Further, the IMS/MMTel approach needs features implemented in the terminals and the legacy CS network in order to handle voice handover to/from the 2G/3G CS telephony service.
Using UMA/GAN over LTE is not a standardized solution so a drawback is that it is a proprietary solution which may make terminal availability a problem. It also adds additional functions to the core/service layer in both the network and terminal, e.g., a GAN controller in the network and GAN protocols in the UE terminal.
The proprietary VoIP approach, if operator controlled, comes with the same drawbacks as for the IMS/MMTel (new core/service layer) approach along with the difficulties associated with it being proprietary and handover to 2G/3G CS may not be supported.
There is yet a further solution for using a legacy CS telephony service with a wireless terminal such as a 3GPP release 8-compliant LTE terminal. In that further solution, also known as a type of Access Division Multiplexing (ADM), transmissions of GSM CS voice are interleaved in between LTE transmissions. See, e.g., PCT/SE2007/000358, which is incorporated herein by reference. In one example implementation of such an ADM solution a wireless terminal simultaneously communicates with two TDMA-based radio systems, e.g., the wireless terminal can maintain communications paths to both systems by means of alternating in time its communication between the two systems. The toggling between the two systems is on a time scale small enough to effectively yield a simultaneous communication between the two systems.
The ADM solution attempts to achieve a good PS connection in parallel with the telephony service when in LTE coverage but still reusing the legacy CS core and deployed GSM network for the telephony service to reduce costs but still maintain good coverage for the telephony service.
The ADM solution may be implemented in several ways. A first example implementation, illustrated in FIG. 1A, is a fully UE-centric solution where no coordination is needed between the GSM CS core and a LTE PS core. A second example implementation, illustrated by FIG. 1B, is a network assisted solution which can either be based on circuit switched fallback (CSFB), or a solution that only reuses paging over LTE.
From a radio perspective, the ADM solution can be realized in any of three different ways: As a first example radio realized embodiment illustrated in FIG. 2A, the LTE transmissions could be multiplexed with the GSM transmissions on a GSM TDMA frame level. In FIG. 2A, frames for GSM transmissions and frames of LTE transmissions have different darkness shading. This first example solution requires that the GSM circuit switched (CS) telephony service only use the half rate codec. When GSM is running at half rate, then every second GSM TDMA frame is not used by the user.
As a second example radio-realized embodiment illustrated in FIG. 2B, the LTE transmissions could be multiplexed with the GSM transmissions on GSM burst level. GSM transmits speech using bursts, each with a duration of 0.577 ms. In speech operation, after having sent one burst, the Rx/Tx part sleeps for 7*0.577 ms until it wakes up again and do a new Rx/Tx process. In this second example this time gap could be used for LTE transmissions.
As a third example radio-realized embodiment illustrated in FIG. 2C, any of above can be used for transmission but by using dual receiver for simultaneous reception of GSM and LTE in the downlink for simplified operation.
The architecture and principles of the circuit switched fallback (CSFB) are defined in, e.g., 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System, Stage 2 (Release 8), which is abbreviated herein as “23.272” and which is incorporated herein by reference in its entirety.
The current circuit switched fallback (CSFB) solution does not allow the wireless terminal (e.g., user equipment unit (UE)) to multiplex transmissions of a circuit switched (CS) voice in Global System for Mobile communication (GSM) with transmissions of a packet switched (PS) session in Long Term Evolution (LTE). For example, the packet switched (PS) session may be moved to GSM as soon as a CS voice call is originated or terminated, as shown in FIG. 3. However, if multiplexing of a circuit switched (CS) call and a packet switched (PS) session is not supported in GSM, e.g. if a Dual Transfer Mode (DTM) is not supported in GSM, the packet switched (PS) session is suspended, as shown in FIG. 4. Recent developments in 3GPP have changed the way that the suspension is executed, but the result is still the same, i.e., no PS connection is possible while the CS call is active (and DTM not supported).
The signalling sequence for the existing circuit switched fallback (CSFB) solution (not using the packet switched (PS) handover alternative) as defined by 3GPP is shown in FIG. 5. FIG. 5 shows, e.g., the originating circuit switched (CS) Call Request in E-UTRAN, Call in GSM Edge Radio Access Network (GERAN)/UTRAN without packet switched (PS) handover (HO) (see, e.g., FIG. 6.3-1 of 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System, Stage 2 (Release 8), which is incorporated by reference herein in its entirety). The sequence in FIG. 5 results in packet switched bearers being moved to GSM (e.g., via a routing area (RA) Update procedure or Combined RA/LA Update procedure in step 6). If CS+PS multiplexing is not supported, e.g., by not supporting DTM in GSM, the PS bearers are also suspended for the duration of the CS call.
Concerning various details of FIG. 5, step 3 generically depicts activities involved with release of the UE (with or without system information). Release of the UE, and hence release of the packet switched (PS) bearers for LTE, occurs either through a Network Assisted Cell Change (e.g., NACC) as depicted by step 3a or through a RRC Connection Release for the target access. Traditionally the Network Assisted Cell Change (e.g., NACC) is an inter-radio access technology (e.g., inter-rat) cell change order message that provides information including system information to the terminal regarding which access and which cell to go to, whereas the RRC Connection Release includes only general re-direct information such as access and frequency. Step 4 of FIG. 5 depicts the eNodeB sending a S1-AP context release request message to the mobility management entity (MME), which results in release of the S1 connection (e.g., a S1 UE context release) as depicted by step 5 of FIG. 5. Step 6 of FIG. 5 depicts performance of the routing area (RA) Update procedure towards the GSM Edge Radio Access Network (GERAN)/UTRANPS Domain. In context of the technology disclosed herein step 6 of FIG. 5 also depicts/applies to a routing area update procedure towards the GSM Edge Radio Access Network (GERAN). Step 7a, 7b, and 8 of FIG. 5 relate to suspension of the GSM packet switched (PS) bearers (if Dual Transfer Mode (DTM) is not supported in GSM). Step 11 of FIG. 5 depicts performance of the routing area (RA) Update (or combined RA/LA Update) to resume the LTE packet switched (PS) bearers after the circuit switched (CS) call is finished if the GSM PS bearers were suspended by step 7a, 7b, and 8. For yet other details concerning FIG. 5, see, e.g., 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System, Stage 2 (Release 8), Chapter 6.5, which is incorporated herein by reference.
FIG. 6 illustrates terminating a CS Call Request in E-UTRAN, Call in GERAN/UTRAN without packet switched (PS) handover (HO) (see, e.g., FIG. 7.4-1 of 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System, Stage 2 (Release 8)). Concerning various details of FIG. 6, step 3 generically depicts activities involved with release of the UE (with or without system information). Release of the UE, and hence release of the packet switched (PS) bearers for LTE, occurs either through a Network Assisted Cell Change (e.g., NACC) as depicted by step 3a or through a RRC Connection Release for the target access. Step 4 of FIG. 6 depicts the eNodeB sending a S1-AP context release request message to the mobility management entity (MME), which results in release of the S1 connection (e.g., a S1 UE context release) as depicted by step 5 of FIG. 6. Step 6 of FIG. 6 depicts performance of the routing area (RA) Update towards the GERAN/UTRAN PS Domain. In context of the technology disclosed herein step 6 of FIG. 5 also depicts/applies to a routing area update towards the GERAN for the Global System for Mobile communication (GSM) (e.g., GERAN (GSM)]. Step 7a, 7b, and 8 of FIG. 6 relate to suspension of the GSM packet switched (PS) bearers (if Dual Transfer Mode (DTM) is not supported in GSM). For more details concerning FIG. 6, see 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System, Stage 2 (Release 8), chapter 7.4, which is incorporated herein by reference. Not shown in FIG. 6, but mentioned in chapter 7.4 of 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System, Stage 2 (Release 8), the wireless terminal performs routing area (RA) Update (or combined RA/LA Update) to resume the LTE PS bearers after the CS call is finished if the PS bearers were suspended by step 7a, 7b, and 8.